US20180284444A1

US20180284444A1 - Display apparatus

Info

Publication number: US20180284444A1
Application number: US15/922,330
Authority: US
Inventors: Hayato MATSUKI; Takashi Takeda
Original assignee: Seiko Epson Corp
Current assignee: Seiko Epson Corp
Priority date: 2017-03-28
Filing date: 2018-03-15
Publication date: 2018-10-04
Anticipated expiration: 2038-03-15
Also published as: JP6852501B2; CN108663808A; US10139634B2; CN108663808B; JP2018165741A

Abstract

There is provided a display apparatus including a light guide system that includes a light-incident portion including a light-incident curved surface on which image light beams as non-parallel light beams are incident and a light-reflective curved surface which reflects the image light beams such that the image light beams are converted into parallel light beams, and a light guide portion being a portion in which a plurality of partial reflection layers are disposed in parallel. The light guide portion is filled with a parallel light flux emitted from the light-incident portion. The light-incident portion is formed of a first transparent member, the portion in which the partial reflection layers are formed is formed of a second transparent member which is surface-bonded to the first transparent member via a bonding surface, and the bonding surface is inclined in the same direction as that of the partial reflection layer.

Description

BACKGROUND

1. Technical Field

The present disclosure relates to a display apparatus including a light guide system.

2. Related Art

As a display apparatus, a configuration including a light guide system in which a light guide portion extends from a light-incident portion on which image light beams are incident has been proposed, and the image light beams which are incident from the light-incident portion are guided to a light-emitting portion via the light guide portion (refer to JP-A-2017-3845). Since a light-incident curved surface and a light-reflective curved surface are formed on the light-incident portion, the image light beams are incident from the light-incident curved surface, are reflected between the light-reflective curved surface and the light-incident curved surface, and then are guided to the light guide portion. On the other hand, a light guide system including a light guide portion in which a plurality of partial reflection layers are disposed parallel to each other has been proposed, and a configuration in which a portion in which the plurality of partial reflection layers are formed and a portion in which a light-incident portion is formed are configured with transparent members different from each other and in which the transparent members are bonded to each other has been proposed (refer to U.S. Pat. No. 9,248,616).
In the display apparatus, it is required that the image light beams emitted from the light-emitting portion have high brightness, and it is required that the light guide portion is made thin because the light guide portion is positioned in front of an eye. However, in JP-A-2017-3845 and U.S. Pat. No. 9,248,616, a configuration for simultaneously satisfying the above requirements is not disclosed. In addition, in U.S. Pat. No. 9,248,616, a configuration in which the light guide system is configured by bonding two transparent members is disclosed. However, U.S. Pat. No. 9,248,616 does not disclose a technique for manufacturing the transparent member in which the plurality of partial reflection layers are formed at a low cost and with a structure suitable for bonding.

SUMMARY

A first advantage of some aspects of the embodiment is to provide a display apparatus capable of emitting image light beams with high brightness from the light-emitting portion even when the light guide portion is made thin.
A second advantage of some aspects of the embodiment is to provide a display apparatus including the transparent member that can be manufactured at a low cost and with a structure suitable for bonding, the transparent member in which the plurality of partial reflection layers are formed.
In order to realize the first advantage, according to an aspect of the embodiment, there is provided a display apparatus including: an image generation system that emits image light beams as non-parallel light beams; and a light guide system that guides the image light beams which are incident, in which the light guide system includes a transparent light-incident portion and a transparent light guide portion, the transparent light-incident portion including a light-incident curved surface on which the image light beams are incident and a light-reflective curved surface which reflects the image light beams incident from the light-incident curved surface, the light-incident curved surface and the light-reflective curved surface convert from the image light beams into a parallel light beams, and the transparent light guide portion being a portion of which one end side in a first direction is connected to the light-incident portion and on which the parallel light beams are incident from the light-incident portion, in which the light guide portion including a first surface extending from the one end side toward another end side in the first direction, and a second surface facing the first surface in parallel in one side of a second direction intersecting with the first direction, the light guide portion has a light-emitting portion provided at a position toward another end side that is away from the light-incident portion, and in which, when viewed from a third direction intersecting with the first direction and the second direction, the image light beams emitted from the same portion of the image generation system are converted into a parallel light flux by the light-incident portion, and the parallel light flux fills the inside of a portion of the light guide portion on the one end side.
In the display apparatus, since the light-incident curved surface and the light-reflective curved surface are formed on the light-incident portion of the light guide system, the image light beams are incident from the light-incident curved surface, are reflected between the light-reflective curved surface and the light-incident curved surface, and then are guided to the light guide portion as parallel light beams. In addition, when a state where the image light beams are emitted as parallel light beams from the light-incident portion to the light guide portion is viewed from the third direction, the entire portion of the light guide portion on the one end side is filled with the image light beams as parallel light beams. Therefore, even when the light guide portion is made thin, the image light beams with high brightness can be emitted from the light-emitting portion.

BRIEF DESCRIPTION OF THE DRAWINGS

The embodiment will be described with reference to the accompanying drawings, wherein like numbers reference like elements.

FIG. 1 is an explanatory view schematically illustrating an example of an appearance of a display apparatus according to a first embodiment.

FIG. 2 is a plan view of an optical system of the display apparatus illustrated in FIG. 1.

FIG. 3 is an explanatory diagram illustrating design examples of a projection lens system and a light-incident portion illustrated in FIG. 2.

FIG. 4 is an explanatory diagram schematically illustrating a state of a parallel light flux in a light guide portion illustrated in FIG. 2.

FIG. 5 is an explanatory diagram illustrating a manufacturing method of the light guide portion illustrated in FIG. 2.

FIG. 6 is an explanatory diagram illustrating another manufacturing method of the light guide portion illustrated in FIG. 2.

FIG. 7 is a plan diagram of the optical system of the display apparatus according to a second embodiment.

FIG. 8 is a plan diagram of the optical system of the display apparatus according to a third embodiment.

FIG. 9 is a plan diagram of the optical system of the display apparatus according to a fourth embodiment.

FIG. 10 is a plan diagram of the optical system of the display apparatus according to a fifth embodiment.

FIG. 11 is a plan diagram of the optical system of the display apparatus according to a sixth embodiment.

FIG. 12 is an explanatory diagram of the light guide portion of the display apparatus according to a seventh embodiment.

FIG. 13 is an explanatory view schematically illustrating an example of an appearance of the display apparatus according to an eighth embodiment.

FIG. 14 is a front view of the display apparatus illustrated in FIG. 13 when viewed from a front direction of an observer.

FIG. 15 is a side view of the display apparatus illustrated in FIG. 13 when viewed from a left side.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

Hereinafter, embodiments according to the invention will be described. In the drawings to be referred to in the following description, in order to make each layer and each member to be recognizable on the drawings, the number and the scale of each layer and each member are made different.

First Embodiment

Overall Configuration

FIG. 1 is an explanatory view schematically illustrating an example of an appearance of a display apparatus 100 according to a first embodiment. The display apparatus 100 illustrated in FIG. 1 is configured with a see-through type eye glass display or the like, and includes a frame 110 provided with temples 111(R) and 111(L) on left and right sides thereof. In the display apparatus 100, a display unit 10 to be described is supported by the frame 110, and an image emitted from the display unit 10 is recognized by a user, as a virtual image. In the present embodiment, the display apparatus 100 includes a right-eye display unit 10(R) and a left-eye display unit 10(L) as the display unit 10. The right-eye display unit 10(R) and the left-eye display unit 10(L) have the same configuration, and are disposed symmetrically in a right-left direction. Thus, in the following description, the right-eye display unit 10(R) will be mainly described, and a description of the left-eye display unit 10(L) will be omitted.
In the following description, a right-left direction is referred to as a first direction X, a front-rear direction is referred to as a second direction Z, and a upper-and-lower direction is referred to as a third direction Y. In addition, one side (right side) in the first direction X is referred to as X1, the other side (left side) in the first direction X is referred to as X2, one side (rear side) in the second direction Z is referred to as Z1, the other side (front side) in the second direction Z is referred to as Z2, one side (upper side) in the third direction Y is referred to as Y1, and the other side (lower side) in the third direction Y is referred to as Y2. Here, since the right-eye display unit 10(R) and the left-eye display unit 10(L) are disposed symmetrically, in the right-eye display unit 10(R) and the left-eye display unit 10(L), one side X1 and the other side X2 in the first direction X are reversed to each other in a right-left direction.

Overall Configuration of Display Unit 10

FIG. 2 is a plan view of an optical system of the display unit 10 illustrated in FIG. 1. In FIG. 2, image light beams L emitted from the center of an image generation system 20 are illustrated by dotted lines, and image light beams L emitted from end portions of the image generation system 20 are illustrated by one-dot chain lines and two-dot chain lines.
As illustrated in FIG. 2, the display unit 10 (right-eye display unit 10(R)) includes an image generation system 20 that emits image light beams L as non-parallel light beams and a light guide system 30 that guides the incident image light beams L to a light-emitting portion 58. In the present embodiment, a projection lens system 70 is disposed between the image generation system 20 and the light guide system 30, and the image light beams L emitted from the image generation system 20 are incident on the light guide system 30 via the projection lens system 70.
In this embodiment, the projection lens system 70 is configured with a first lens 71 and a second lens 72 disposed between the first lens 71 and the light guide system 30.
The light guide system 30 includes a transparent light-incident portion 40 that includes a light-incident curved surface 41 on which the image light beams L are incident and a light-reflective curved surface 42 which reflects the image light beams L incident from the light-incident curved surface 41 between the light-incident curved surface 41 and the light-reflective curved surface 42 such that the reflected image light beams L are converted into parallel light beams, and a transparent light guide portion 50 of which one end 51 side in the first direction X (end portion in one side X1) is connected to the light-incident portion 40. The light guide portion 50 guides the parallel light beams which are incident from the light-incident portion 40, to the light-emitting portion 58. The light guide portion 50 includes a first surface 56 extending from the one end 51 side in one side X1 toward the other end 52 side in the first direction X (end portion in the other side X2), a second surface 57 facing the first surface 56 in parallel in one side Z1 of the second direction Z intersecting with the first direction X, and the light-emitting portion 58 which is provided at a portion of the second surface 57 that is away from the light-incident portion 40. In the present embodiment, the light-emitting portion 58 is a portion of the second surface 57 that is away from the light-incident portion 40 in the other side X2 of the first direction X. In the light guide system 30 with this configuration, the parallel light beams emitted from the light-incident portion 40 to the light guide portion 50 are reflected between the first surface 56 and the second surface 57, are guided to the light-emitting portion 58 from one side X1 to the other side X2 in the first direction X, and are emitted from the light-emitting portion 58.

Detailed Configuration of Image Generation System 20

The image generation system 20 is a light modulation panel configured with a liquid crystal device, an organic electroluminescence device, or the like, and is disposed so as to face the light-incident curved surface 41 in one side X1 of the first direction X and in one side Z1 of the second direction Z with respect to the light-incident portion 40. In the present embodiment, the image generation system 20 is disposed obliquely such that an end portion 21 thereof in one side X1 of the first direction X is positioned toward the other side Z2 of the second direction Z than the other end portion 22 thereof in the other side X2 of the first direction X is.

Detailed Configuration of Light-Incident Portion 40

In the light-incident portion 40, the light-incident curved surface 41 is a surface toward one side Z1 of the second direction Z, and faces the image generation system 20 via the projection lens system 70. Thus, the light-incident curved surface 41 is disposed obliquely such that an end portion 411 in the other side Z2 of the second direction Z is positioned toward one side X1 of the first direction X than an end portion 412 in one side Z1 of the second direction Z is. On the other hand, a distance between the end portion 412 of the light-incident curved surface 41 and the end portion 22 of the image generation system 20 is wider than a distance between the end portion 411 of the light-incident curved surface 41 and the end portion 21 of the image generation system 20. The light-incident curved surface 41 is made of a curved surface such as an aspherical surface or a free curved surface, and in the present embodiment, the light-incident curved surface 41 is made of a convex-shaped free curved surface. Although a reflection film or the like is not formed on the light-incident curved surface 41, the light-incident curved surface 41 totally reflects light beams which are incident at an incident angle equal to or larger than a critical angle. Thus, the light-incident curved surface 41 has transparency and reflectivity.
The light-reflective curved surface 42 is a surface toward the other side Z2 of the second direction Z, and is disposed obliquely such that an end portion 421 in one side X1 of the first direction X is positioned toward one side Z1 of the second direction Z than an end portion 422 in the other side X2 of the first direction X is. The light-reflective curved surface 42 is made of a curved surface such as an aspherical surface or a free curved surface, and in the present embodiment, the light-reflective curved surface 42 is made of a convex-shaped free curved surface. The light-reflective curved surface 42 may employ a configuration in which a reflection film or the like is not formed and light beams incident at an incident angle equal to or larger than a critical angle are totally reflected. Here, in the present embodiment, the light-reflective curved surface 42 is provided with a reflective metal layer 420 including aluminum, silver, magnesium, chromium, or the like as a main component.
In the light-incident portion 40 with this configuration, when the image light beams L as non-parallel light beams are incident on the light-incident curved surface 41, the image light beams L incident from the light-incident curved surface 41 are refracted by the light-incident curved surface 41, and are directed toward the light-reflective curved surface 42. Next, the image light beams L are reflected by the light-reflective curved surface 42, and are directed again toward the light-incident curved surface 41. At this time, since the image light beams L are incident on the light-incident curved surface 41 at an incident angle equal to or larger than a critical angle, the image light beams L are reflected by the light-incident curved surface 41 toward the light guide portion 50. Meanwhile, the image light beams L are converted into parallel light beams.

Design Examples of Projection Lens System 70 and Light-Incident Portion 40

FIG. 3 is an explanatory diagram illustrating design examples of the projection lens system 70 and the light-incident portion 40 illustrated in FIG. 2. The first lens 71, the second lens 72, the light-incident curved surface 41, and the light-reflective curved surface 42 are configured as illustrated in FIG. 3. In an upper portion of FIG. 3, coefficients of a free curved surface which is represented by Equation 1 illustrated in FIG. 3 are indicated. In a lower portion of FIG. 3, a refractive index (nd) and an Abbe number (υ) of a material that constitutes the first lens 71, the second lens 72, and the light-incident portion 40 are indicated. Among surface numbers (3) to (8) illustrated in FIG. 3, a surface number (3) corresponds to the light-incident curved surface 41, and a surface number (4) corresponds to the light-reflective curved surface 42. A surface number (5) corresponds to a lens surface of the second lens 72 on a light-emitting side, and a surface number (6) corresponds to a lens surface of the second lens 72 on a light-incident side. A surface number (7) corresponds to a lens surface of the first lens 71 on a light-emitting side, and a surface number (8) corresponds to a lens surface of the first lens 71 on a light-incident side.

Detailed Configuration of Light Guide Portion 50

In the light guide portion 50, the first surface 56 and the second surface 57 are disposed in parallel, and a thickness between the first surface 56 and the second surface 57 in the second direction Z (a dimension of the light guide portion 50 in the second direction Z) is thinner than a dimension of the light-incident portion 40 in the second direction Z. The first surface 56 and the second surface 57 totally reflect the light beams which are incident at an incident angle equal to or larger than a critical angle, according to a difference in refractive index between the light guide portion 50 and the outside (air). Therefore, a reflection film or the like is not formed on the first surface 56 (a first reflection surface) and the second surface 57 (a second reflection surface).
In a portion of the light guide portion 50 that is away from the light-incident portion 40, a plurality of partial reflection layers 55 are disposed parallel to each other along the first direction X, the partial reflection layers 55 being inclined toward one side X1 of the first direction X from a normal direction with respect to the second surface 57 when viewed from the third direction Y. The light-emitting portion 58 is a portion of the second surface 57 that overlaps with the plurality of partial reflection layers 55 in the first direction X, and is a region with a predetermined width in the first direction X.
Each of the plurality of partial reflection layers 55 is made of a dielectric multilayer film. In addition, at least one of the plurality of partial reflection layers 55 may be a composite layer of a dielectric multilayer film and a reflective metal layer (thin film) including aluminum, silver, magnesium, chromium, or the like as a main component. In a case where the partial reflection layer 55 is configured to include a metal layer, there is an effect in that a reflectance of the partial reflection layer 55 can be increased, and in that an incident angle dependence and a polarization dependence on a transmittance and a reflectance of the partial reflection layer 55 can be optimized.
In the light guide portion 50 with this configuration, the image light beams L as parallel light beams which are incident from the light-incident portion 40 are reflected between the first surface 56 and the second surface 57, and travel from one side X1 to the other side X2 in the first direction X. A part of the image light beams L incident on the partial reflection layer 55 is reflected by the partial reflection layer 55, and is emitted from the light-emitting portion 58 toward an eye E of an observer. In addition, the rest of the image light beams L incident on the partial reflection layer 55 pass through the partial reflection layer 55, and are incident on the next partial reflection layer 55 which is adjacent to the partial reflection layer 55 in the other side X2 of the first direction X. Thus, in each of the plurality of partial reflection layers 55, the image light beams L reflected toward one side Z1 of the second direction Z are emitted from the light-emitting portion 58 toward the eye E of the observer. Therefore, the observer can recognize a virtual image. At that time, when light beams are incident from the outside to the light guide portion 50, the light beams are incident on the light guide portion 50, pass through the partial reflection layer 55, and reach the eye E of the observer. Therefore, the observer can see the image generated by the image generation system 20, and can see a scenery or the like of the outside in a see-through manner.

State of Parallel Light Flux in Light Guide Portion 50

FIG. 4 is an explanatory diagram schematically illustrating a state of a parallel light flux in the light guide portion 50 illustrated in FIG. 2, and schematically illustrates a state where a non-parallel light flux is converted into a parallel light flux L0 (image light beam L) by the light-incident portion 40 and then the parallel light flux L0 is viewed from the third direction Y, the non-parallel light flux being incident on the light-incident curved surface 41 of the light-incident portion 40 from the same portion of the image generation system 20 via the projection lens system 70. In FIG. 4, a center light beam L0 c of the parallel light flux L0 is illustrated by a solid line, a first light beam L0 a which is positioned at one end portion of the parallel light flux L0 is illustrated by a long broken line, and a second light beam L0 b which is positioned at the other end portion of the parallel light flux L0 is illustrated by a one-dot chain line. In addition, in an upper portion of FIG. 4, a first parallel light flux portion L01 interposed between the center light beam L0 c and the first light beam L0 a is illustrated by a gray color, and in a lower portion of FIG. 4, a second parallel light flux portion L02 interposed between the center light beam L0 c and the second light beam L0 b is illustrated by a gray color.
As illustrated in FIG. 4, in the display unit 10, when the non-parallel light flux is converted into a parallel light flux L0 by the light-incident portion 40 and then the parallel light flux L0 is viewed from the third direction Y, the non-parallel light flux being incident on the light-incident curved surface 41 of the light-incident portion 40 from the same portion of the image generation system 20, the inside of one end portion (a portion in one side X1 of the first direction X) of the light guide portion 50 at which the light-incident portion 40 is positioned is filled with the parallel light flux L0. More specifically, in the parallel light flux L0, when the first parallel light flux portion L01 (gray portion) interposed between the center light beam L0 c and the first light beam L0 a is combined with the second parallel light flux portion L02 (gray portion) interposed between the center light beam L0 c and the second light beam L0 b of the parallel light flux L0, the inside of the light guide portion 50 is filled with the parallel light flux L0.
For example, assuming that the first surface 56 and the second surface 57 are parallel to each other, that a light flux diameter of the parallel light flux L0 is D, that a distance between the first surface 56 and the second surface 57 in the second direction Z is t, and that an incident angle of the parallel light flux L0 with respect to the first surface 56 and the second surface 57 is θ, such a configuration can be realized by setting the light flux diameter D, the distance t, and the incident angle θ so as to satisfy the following conditional equation.
D=2t×Sin θ
In other words, positions at which the first light beam L0 a and the second light beam L0 b are incident on the second surface 57 are set so as to be positioned on an imaginary normal line Ls at a position at which the center light beam L0 c is incident on the first surface 56, and thus the configuration can be realized. As illustrated in FIG. 2, although the image light beams L are emitted from each portion of the image generation system 20, in the present embodiment, the image light beams L emitted from each portion of the image generation system 20 satisfy the above condition.

Bonding Structure

Referring to FIG. 2 again, in the present embodiment, the light-incident portion 40 is formed of a first transparent member 61, a portion 54 of the light guide portion 50 at which at least the plurality of partial reflection layers 55 are formed is formed of a second transparent member 62 which is surface-bonded to the first transparent member 61 via a bonding surface 63 (first bonding surface) in the first direction X. Therefore, the bonding surface 63 is positioned between the portion 54 at which the plurality of partial reflection layers 55 are formed and the light-incident portion 40. In this configuration, the light-incident portion 40 and the partial reflection layers 55 can be manufactured by an appropriate method. For example, the light-incident portion 40 is configured with a resin molded component made of a cycloolefin polymer or the like, while the portion 54 of the light guide portion 50 at which the plurality of partial reflection layers 55 are formed is formed, as illustrated in FIGS. 5 and 6, by stacking transparent substrates with the partial reflection layers 55 interposed therebetween and cutting the transparent substrates.
In the present embodiment, the bonding surface 63 is disposed between the light guide portion 50 and the light-incident portion 40. Here, a first end portion 631 of the bonding surface 63 in the other side Z2 of the second direction Z is positioned between the first surface 56 and a reflection region of the light-incident portion 40, and a second end portion 632 of the bonding surface 63 in one side Z1 of the second direction Z is positioned between the second surface 57 and the reflection region of the light-incident portion 40. More specifically, the second end portion 632 of the bonding surface 63 is positioned between the region (reflection region) of the light-incident curved surface 41 on which the image light beams L are incident from the light-reflective curved surface 42 and the second surface 57. In addition, the first end portion 631 of the bonding surface 63 is positioned between the region (reflection region) of the light-reflective curved surface 42 on which the image light beams L are incident from the light-incident curved surface 41 and the first surface 56. Therefore, the bonding surface 63 is less likely to hinder appropriate reflection in the light-incident portion 40 and appropriate reflection in the light guide portion 50.
In addition, the first end portion 631 of the bonding surface 63 is positioned closer to the one end 51 side than the second end portion 632 is such that a flat surface is made. Therefore, the bonding surface 63 is inclined in the same direction as that of the partial reflection layer 55. In addition, in the other end 52 (third surface), a first edge portion 521 on the other side Z2 of the second direction Z is positioned closer to the one end 51 side than a second edge portion 522 on one side Z1 of the second direction Z is. Therefore, the other end 52 is inclined in the same direction as that of the partial reflection layer 55.

Manufacturing Method of Light Guide Portion 50

FIG. 5 is an explanatory diagram illustrating a manufacturing method of the light guide portion 50 illustrated in FIG. 2. FIG. 6 is an explanatory diagram illustrating another manufacturing method of the light guide portion 50 illustrated in FIG. 2. In order to manufacture the light guide portion 50 (second transparent member 62) illustrated in FIG. 2, as illustrated in FIG. 5, a plurality of transparent substrates 66, each of which includes the partial reflection layer 55 formed on one side thereof, are overlapped while being shifted, and in this state, the transparent substrates 66 are bonded to each other via a bonding layer while applying a load to the transparent substrates 66, thereby forming a stacked body 67. At this time, the transparent substrates 66 on which the partial reflection layers 55 are not formed are also stacked. Next, the stacked body 67 is cut obliquely. As a result, the light guide portion 50 (second transparent member 62) is obtained. At this time, since the first surface 56 and the second surface 57 are formed by cut surfaces, polishing or the like is performed on the cut surfaces. The transparent substrates 66 are glass substrates, quartz substrates, resin substrates, or the like. In a case where the transparent substrates 66 are glass substrates, the transparent substrates 66 are bonded to each other by glass bonding or the like, and thus there is no need to use an adhesive. In addition, the transparent substrates 66 may be bonded to each other using an adhesive.
On the other hand, in a method illustrated in FIG. 6, a plurality of transparent substrates 66, each of which includes the partial reflection layer 55 formed on one side thereof, are overlapped without being shifted, and in this state, the transparent substrates 66 are bonded to each other via a bonding layer while applying a load to the transparent substrates 66, thereby forming a stacked body 68. At this time, the transparent substrates 66 on which the partial reflection layers 55 are not formed are also stacked. Next, the stacked body 68 is cut obliquely. As a result, the light guide portion 50 (second transparent member 62) is obtained. At this time, since the first surface 56 and the second surface 57 are formed by cut surfaces, polishing or the like is performed on the cut surfaces. The transparent substrates 66 are glass substrates, quartz substrates, resin substrates, or the like. In a case where the transparent substrates 66 are glass substrates, the transparent substrates 66 are bonded to each other by glass bonding or the like, and thus there is no need to use an adhesive. In addition, the transparent substrates 66 may be bonded to each other using an adhesive.
According to the method illustrated in FIG. 5, it is possible to reduce a loss in material. On the other hand, in the method illustrated in FIG. 6, since the plurality of transparent substrates 66 are entirely overlapped, when curing the adhesive while applying a load to the transparent substrates 66, by the method illustrated in FIG. 5, a uniform load is applied to the entire transparent substrates 66. Therefore, the transparent substrates 66 can be bonded to each other in a state of being in close contact with each other uniformly.
In addition, the bonding surface 63 and the other end 52 are formed by the transparent substrates 661 and 662 which are stacked between both ends of the light guide portion 50. Therefore, the thickness of the transparent substrate 661 is set corresponding to a distance between the bonding surface 63 and the partial reflection layer 55, and the thickness of the transparent substrate 662 is set corresponding to a distance between the other end 52 and the partial reflection layer 55. Here, the bonding surface 63 and the other end 52 are inclined in the same direction as that of the partial reflection layer 55. Therefore, even in a case where the bonding surface 63 is formed by processing the transparent substrate 661 positioned on one end portion of the plurality of transparent substrates 66, unlike a case where the bonding surface 63 is inclined in a direction opposite to the partial reflection layer 55 (a case illustrated by a two-dot chain line L63), it is not necessary to make the transparent substrate 661 excessively thick. In addition, even in a case where the other end 52 is formed by processing the transparent substrate 662 positioned on the other end portion of the plurality of transparent substrates 66, unlike a case where the other end 52 is inclined in a direction opposite to the partial reflection layer 55 (a case illustrated by a two-dot chain line L520), it is not necessary to make the transparent substrate 662 excessively thick. Therefore, the light guide portion 50 (second transparent member 62) including the bonding surface 63 and the other end 52 formed on both ends thereof can be connected to the light-incident portion 40 at a low cost, and thus it is possible to reduce a cost of the display unit 10 (display apparatus 100).
Particularly, in a case where the bonding surface 63 and the other end 52 are parallel to the partial reflection layer 55, processing such as polishing may be performed on the transparent substrates 661 and 662, and there is no need to perform angle adjustment processing. Therefore, the light guide portion 50 (second transparent member 62) including the bonding surface 63 and the other end 52 (third surface) formed on both ends thereof can be connected to the light-incident portion 40 at a low cost, and thus it is possible to reduce a cost of the display apparatus 100.

Main Effect of Present Embodiment

As described above, in the display apparatus 100 (display unit 10) according to the present embodiment, since the light-incident curved surface 41 and the light-reflective curved surface 42 are formed on the light-incident portion 40 of the light guide system 30, the image light beams L are incident from the light-incident curved surface 41, are reflected between the light-reflective curved surface 42 and the light-incident curved surface 41, and then are guided to the light guide portion 50 as parallel light beams. Therefore, design requirements for the projection lens system 70 can be relaxed, and thus, it is possible to simplify the projection lens system 70. For example, the projection lens system 70 can be configured with two lenses (the first lens 71 and the second lens 72). In addition, since the projection lens system 70 can be simplified, it is possible to reduce a size and a weight of the display apparatus 100 (display unit 10).
In addition, in the display apparatus 100 (display unit 10), when the non-parallel light flux is converted into a parallel light flux L0 by the light-incident portion 40 and then the parallel light flux L0 is viewed from the third direction Y, the non-parallel light flux being incident on the light-incident curved surface 41 of the light-incident portion 40 from the same portion of the image generation system 20, the inside of the portion of the light guide portion 50 in one side X1 of the first direction X is filled with the parallel light flux L0. Therefore, even when the light guide portion 50 is made so as to have a small thickness in the second direction Z, the observer can recognize a virtual image with high brightness.
In addition, in the light guide portion 50, the plurality of partial reflection layers 55 are disposed along the first direction X in parallel with each other, and the light-emitting portion 58 is a portion at which the second surface 57 overlaps with the plurality of partial reflection layers 55 in the first direction X. Therefore, since the light-emitting portion 58 is a region with a predetermined width in the first direction X, even in a case where a position of the eye E of the observer moves slightly in the first direction X, the observer can recognize a virtual image.
In addition, in the light-incident portion 40, the light-reflective curved surface 42 is provided with the metal layer 420, and thus, even in a case where incident angles of the light beams with respect to the light-reflective curved surface 42 are small, the light-reflective curved surface 42 can reliably reflect the light beams. Therefore, the observer can recognize a virtual image with high brightness.
In addition, the light-incident portion 40 is formed of a first transparent member 61, a portion 54 of the light guide portion 50 at which at least the plurality of partial reflection layers 55 are formed is formed of a second transparent member 62 which is surface-bonded to the first transparent member 61 via a bonding surface 63 (first bonding surface) in the first direction X. Therefore, the light-incident portion 40 and the partial reflection layers 55 can be manufactured by an appropriate method. For example, the light-incident portion 40 is configured with a resin molded component, while the plurality of partial reflection layers 55 of the light guide portion 50 are manufactured by the method described with reference to FIGS. 5 and 6. In addition, since the bonding surface 63 and the other end 52 are inclined in the same direction as that of the partial reflection layer 55, as described with reference to FIGS. 5 and 6, it is possible to reduce a cost of the display unit 10 (display apparatus 100).

Second Embodiment

FIG. 7 is a plan diagram of the optical system of the display apparatus 100 according to a second embodiment. Since the basic configuration of the present embodiment and each embodiment to be described is the same as that of the first embodiment, the same reference numerals are given to the common portions, and a description thereof will be omitted.
As illustrated in FIG. 7, even in the present embodiment, similar to the first embodiment, the light-incident portion 40 is formed of the first transparent member 61, the portion 54 of the light guide portion 50 at which at least the plurality of partial reflection layers 55 are formed is formed of the second transparent member 62 which is surface-bonded to the first transparent member 61 via the bonding surface 63 (first bonding surface) in the first direction X. In the present embodiment, the bonding surface 63 is disposed at a portion between the plurality of partial reflection layers 55 and one end 51 in the light guide portion 50. In addition, even in the present embodiment, similar to the first embodiment, the bonding surface 63 and the other end 52 are inclined in the same direction as that of the partial reflection layer 55. Other configurations are the same as in the first embodiment. Therefore, the same effect can be obtained as that of the first embodiment.

Third Embodiment

FIG. 8 is a plan diagram of the optical system of the display apparatus 100 according to a third embodiment. As illustrated in FIG. 8, even in the present embodiment, similar to the first embodiment, the light-incident portion 40 is formed of the first transparent member 61, the portion 54 of the light guide portion 50 at which at least the plurality of partial reflection layers 55 are formed is formed of the second transparent member 62 which is surface-bonded to the first transparent member 61 via the bonding surface 63 (first bonding surface) in the first direction X. In the present embodiment, the bonding surface 63 is positioned at a position overlapping with the partial reflection layer 55 positioned closest to the one end 51 side among the plurality of partial reflection layers 55. Therefore, similar to the first embodiment, the bonding surface 63 is inclined in the same direction as that of the partial reflection layer 55. Other configurations are the same as in the first embodiment. Therefore, the same effect can be obtained as that of the first embodiment.

Fourth Embodiment

FIG. 9 is a plan diagram of the optical system of the display apparatus 100 according to a fourth embodiment. As illustrated in FIG. 9, even in the present embodiment, similar to the first embodiment, the light-incident portion 40 is formed of the first transparent member 61, the portion 54 of the light guide portion 50 at which at least the plurality of partial reflection layers 55 are formed is formed of the second transparent member 62 which is surface-bonded to the first transparent member 61 via the bonding surface 63 (first bonding surface) in the first direction X. In the present embodiment, the bonding surface 63 is positioned between the light guide portion 50 and the light-incident portion 40. Here, the bonding surface 63 includes a plurality of stepped portions 630. Therefore, an area of the bonding surface 63 is large, and thus a bonding strength between the first transparent member 61 and the second transparent member 62 can be increased. Even in this case, since the first end portion 631 of the bonding surface 63 is positioned closer to the one end 51 side than the second end portion 632 is, the same effect can be obtained as in the case where the bonding surface 63 is inclined in the same direction as that of the partial reflection layer 55. Other configurations are the same as in the first embodiment. Therefore, the same effect can be obtained as that of the first embodiment.

Fifth Embodiment

FIG. 10 is a plan diagram of the optical system of the display apparatus 100 according to a fifth embodiment. As illustrated in FIG. 10, even in the present embodiment, similar to the first embodiment, the light-incident portion 40 is formed of the first transparent member 61, the portion 54 of the light guide portion 50 at which at least the plurality of partial reflection layers 55 are formed is formed of the second transparent member 62 which is surface-bonded to the first transparent member 61 via the bonding surface 63 (first bonding surface) in the first direction X. In the present embodiment, the bonding surface 63 is positioned between the light guide portion 50 and the light-incident portion 40. In addition, the bonding surface 63 is inclined in the same direction as that of the partial reflection layer 55.
Here, the first transparent member 61 includes a plate-shaped portion 610 overlapping with the light guide portion 50 in one side Z1 of the second direction Z, and the plate-shaped portion 610 is surface-bonded to the light guide portion 50 via a bonding surface 64 (a second bonding surface) in the second direction Z. Therefore, a bonding area between the first transparent member 61 and the second transparent member 62 is large, and thus a bonding strength between the first transparent member 61 and the second transparent member 62 can be increased. Other configurations are the same as in the first embodiment. Therefore, the same effect can be obtained as that of the first embodiment.

Sixth Embodiment

FIG. 11 is a plan diagram of the optical system of the display apparatus 100 according to a sixth embodiment. As illustrated in FIG. 11, even in the present embodiment, similar to the first embodiment, the light-incident portion 40 is formed of the first transparent member 61, the portion 54 of the light guide portion 50 at which at least the plurality of partial reflection layers 55 are formed is formed of the second transparent member 62 which is surface-bonded to the first transparent member 61 via the bonding surface 63 (first bonding surface) in the first direction X. In the present embodiment, the bonding surface 63 is positioned at a position overlapping with the partial reflection layer 55 positioned closest to the one end 51 side among the plurality of partial reflection layers 55.
Here, the first transparent member 61 includes the plate-shaped portion 610 overlapping with the light guide portion 50 in one side Z1 of the second direction Z, and the plate-shaped portion 610 is surface-bonded to the light guide portion 50 via the bonding surface 64 (a second bonding surface) in the second direction Z. Therefore, a bonding area between the first transparent member 61 and the second transparent member 62 is large, and thus a bonding strength between the first transparent member 61 and the second transparent member 62 can be increased. Other configurations are the same as in the first embodiment. Therefore, the same effect can be obtained as that of the first embodiment.

Seventh Embodiment

FIG. 12 is an explanatory diagram of the light guide portion 50 of the display apparatus 100 according to a seventh embodiment. In the present embodiment, the partial reflection layer 55 formed in the light guide portion 50 illustrated in FIG. 2 has a characteristic in that a transmittance and a reflectance change according to the incident angle. In the present embodiment, each of the plurality of partial reflection layers 55 has a larger reflectance when the incident angle is large than when the incident angle is small.
In the present embodiment, an angle formed by the partial reflection layer 55 and the second surface 57 is 45° to 70°. In addition, when the incident angle with respect to the partial reflection layer 55 is small, the transmittance is 98% or more, and the reflectance is 2% or less. On the other hand, when the incident angle with respect to the partial reflection layer 55 is large, the transmittance is 77%, and the reflectance is 23%. In this configuration, as described with reference to Tables 1, 2 and 3, a variation in brightness of the image light beams L emitted from each position of the light-emitting portion 58 in the first direction X can be reduced. More specifically, although light-emitting intensity of the image light beams L decreases in the light-emitting portion 58 as a distance from the light-incident portion 40 increases in the first direction X, according to the present embodiment, the decrease in the light-emitting intensity can be suppressed.

TABLE 1

	Number of transmissions	Number of

		Trans-	Trans-	reflections
		mittance	mittance	At large
		of 77%	of 98%	incident
Light-		At large	At small	angle And
emitting	Light	incident	incident	reflectance	Brightness
position	beam	angle	angle	of 23%	(%)

Portion	L11		0	0	1	23.0
55a	L12		0	0	1	23.0
	L13	0	0	1	23.0
Portion	L11		0	1	1	22.5
55b	L12		1	0	1	17.7
	L13	1	0	1	17.7
Portion	L11		0	2	1	22.1
55c	L12		1	1	1	17.4
	L13	2	0	1	13.6
Portion	L11		0	3	1	21.6
55d	L12		1	2	1	17.0
	L13	2	1	1	13.4
Portion	L11		1	3	1	16.7
55e	L12		1	3	1	16.7
	L13	2	2	1	13.1

TABLE 2

		Number	Number
		of trans-	of reflec-
		missions	tions
Light-		At trans-	At reflec-	Bright-
emitting	Light	mittance	tance	ness
position	beam	of 77%	of 23%	(%)

Portion	L11		0	1	23.0
55a	L12		0	1	23.0
	L13	0	1	23.0
Portion	L11		1	1	17.7
55b	L12		1	1	17.7
	L13	1	1	17.7
Portion	L11		2	1	13.6
55c	L12		2	1	13.6
	L13	2	1	13.6
Portion	L11		3	1	10.5
55d	L12		3	1	10.5
	L13	3	1	10.5
Portion	L11		4	1	8.1
55e	L12		4	1	8.1
	L13	4	1	8.1

TABLE 3

		Number of	Number of
		transmissions	reflections
Light-		At trans-	At reflec-	Bright-
emitting	Light	mittance	tance	ness
position	beam	of 77%	of 23%	(%)

Portion 55a	L11		0	1	23.0
	L12	0	1	23.0
	L13	0	1	23.0
Portion 55b	L11		0	1	23.0
	L12	1	1	17.7
	L13	1	1	17.7
Portion 55c	L11		0	1	23.0
	L12	0	1	17.7
	L13	2	1	13.6
Portion 55d	L11		0	1	23.0
	L12	1	1	17.7
	L13	2	1	13.6
Portion 55e	L11		0	1	23.0
	L12	1	1	17.7
	L13	2	1	13.6

For example, in a case where the brightness (light-emitting intensity) of the light beams emitted from each portion between the partial reflection layers 55 illustrated in FIG. 2 is calculated, as shown in a result of Table 1, a variation in brightness of the image light beams L emitted from each position of the light-emitting portion 58 in the first direction X can be reduced. Table 1 shows the number of transmissions and the number of reflections until the image light beams L are emitted from portions 55 a, 55 b, 55 c, 55 d, and 55 e between the partial reflection layers 55 illustrated in FIG. 12. The number of reflections when the image light beams L are finally reflected at a large incident angle is shown. On the other hand, in a case where the incident angle is large and a case where the incident angle is small, there is a difference in the number of transmissions. In the present embodiment, the transmittance at the partial reflection layer 55 differs depending on the incident angle. Therefore, the number of transmissions is shown by being divided into a case where the incident angle is large and a case where the incident angle is small. In addition, among the light beams emitted from the portions 55 a, 55 b, 55 c, 55 d, and 55 e illustrated in FIG. 12, Table 1 shows brightness of the light beam L11 emitted from the portion positioned closest to one side X1 of the first direction X, the light beam L12 emitted from the portion positioned at the center in the first direction X, and the light beam L13 emitted from the portion positioned closest to the other side X2 of the first direction X.
As can be seen from Table 1, the brightness in each of the portions 55 a, 55 b, 55 c, 55 d, and 55 e is from 13.1% to 23%, and a variation in brightness is small. In addition, a variation in brightness of the light beams L11, L12, and L13 emitted from each of the portions 55 a, 55 b, 55 c, 55 d, and 55 e is small. On the other hand, in a case where the transmittance at the partial reflection layer 55 is 77% or more and the reflectance at the partial reflection layer 55 is 23% regardless of the incident angle with respect to the partial reflection layer 55, the brightness of each of the light beams L11, L12, and L13 emitted from each of the portions 55 a, 55 b, 55 c, 55 d, and 55 e is shown in Table 2. As can be seen from Table 2, the brightness of each of the light beams L11, L12, and L13 emitted from each of the portions 55 a, 55 b, 55 c, 55 d, and 55 e is from 8.1% to 23%, and a variation in brightness is large.
In addition, as illustrated in FIG. 12, in a case where a light-transmitting layer 59 in which the partial reflection layers 55 are not formed is provided on the light guide portion 50, in which the partial reflection layers 55 are formed, on the other side Z2 of the second direction Z, the number of transmissions through the partial reflection layers 55 can be reduced. Therefore, in a case where the transmittance at the partial reflection layer 55 is 77% or more and the reflectance at the partial reflection layer 55 is 23% regardless of the incident angle with respect to the partial reflection layer 55, the brightness of each of the light beams L11, L12, and L13 emitted from each of the portions 55 a, 55 b, 55 c, 55 d, and 55 e is shown in Table 3. As can be seen from Table 3, the brightness of each of the light beams L11, L12, and L13 emitted from each of the portions 55 a, 55 b, 55 c, 55 d, and 55 e is from 13.1% to 23%, and a variation in brightness is small. Here, in this case, the thickness of the light guide system 30 in the second direction Z is increased by the provision of the light-transmitting layer 59.
Therefore, as in the present embodiment, instead of providing the light-transmitting layer 59 (refer to FIG. 12) in which the partial reflection layers 55 are not formed, the light guide portion 50 is formed only by the portion in which the partial reflection layers 55 are formed, and thus, even in a case where the thickness of the light guide portion 50 is decreased, it is possible to reduce a difference in brightness of the image light beams L emitted from a position of the light-emitting portion 58 away from the light-incident portion 40 in the first direction X. In addition, a variation in brightness of the light beams L11, L12, and L13 emitted from each of the portions 55 a, 55 b, 55 c, 55 d, and 55 e can be reduced. Further, since unnecessary reflection at the partial reflection layers 55 is reduced, it is possible to suppress a deterioration in display quality due to occurrence of ghost.

Eighth Embodiment

In the seventh embodiment, the partial reflection layer 55 has a larger reflectance when the incident angle with respect to the partial reflection layer 55 is large than when the incident angle with respect to the partial reflection layer 55 is small. On the other hand, for example, in a case where an angle formed by the partial reflection layer 55 and the second surface 57 is 25° to 40°, a configuration in which the partial reflection layer 55 has a larger reflectance when the incident angle with respect to the partial reflection layer 55 is small than when the incident angle with respect to the partial reflection layer 55 is large, may be adopted.

Ninth Embodiment

FIG. 13 is an explanatory view schematically illustrating an example of an appearance of the display apparatus 100 according to a ninth embodiment. FIG. 14 is a front view of the display apparatus 100 illustrated in FIG. 13 when viewed from a front direction of the observer. FIG. 15 is a side view of the display apparatus 100 illustrated in FIG. 13 when viewed from a left side. In the display apparatus 100 according to the first embodiment, the image generation system 20 and the light-incident portion 40 are disposed on an ear side, and the image light beams L are guided from the ear side toward a nose side. On the other hand, in the present embodiment, as illustrated in FIGS. 13, 14, and 15, the image generation system 20 and the light-incident portion 40 are disposed in front of the eye and above the eye, and the light guide portion 50 extends from an upper side toward a lower side. Thus, the image light beams L are guided from the upper side toward the lower side, and are emitted toward the eye. Therefore, in the present embodiment, an upper-lower direction corresponds to the first direction X, a front-rear direction corresponds to the second direction Z, and a right-left direction corresponds to the third direction Y. In addition, one side X1 of the first direction X corresponds to the upper side, and the other side X2 of the first direction X corresponds to the lower side.

OTHER EMBODIMENTS

In the embodiments described above, although the image light beams are emitted from the light-emitting portion 58 of the light guide portion 50 by partial reflection on the plurality of partial reflection layers 55, the embodiment may be applied to the display apparatus 100 (display unit 10) with a configuration in which a diffraction element is provided in the light-emitting portion 58 and the image light beams are emitted by the diffraction element.
In the embodiments described above, an anti-reflection film may be provided on the first surface 56 or the second surface 57. Even in a case where light beams from the outside (see-through light beams) are transmitted or a case where the image light beams L are emitted from the light guide portion 50 to the outside, when reflection occurs at an interface between the light guide portion 50 and the light-incident portion 40, a loss in the light beams occurs due to the reflection. Therefore, by providing the anti-reflection film, it is possible to increase the brightness for both of the see-through light beams and the image light beams L. In addition, an amount of unnecessary return light beams can be decreased, and thus occurrence of ghost can be suppressed.
The entire disclosure of Japanese Patent Application No. 2017-062397, filed Mar. 28, 2017 is expressly incorporated by reference herein.

Claims

What is claimed is:

1. A display apparatus comprising:

an image generation system that emits image light beams as non-parallel light beams; and

a light guide system that guides the image light beams which are incident,

wherein the light guide system includes a transparent light-incident portion and a transparent light guide portion, the transparent light-incident portion including a light-incident curved surface on which the image light beams are incident and a light-reflective curved surface which reflects the image light beams incident from the light-incident curved surface, the light-incident curved surface and the light-reflective curved surface convert from the image light beams into a parallel light beams, and the transparent light guide portion being a portion of which one end side in a first direction is connected to the light-incident portion and on which the parallel light beams are incident from the light-incident portion,

wherein the light guide portion including:

a first surface extending from the one end side toward another end side in the first direction; and

a second surface facing the first surface in parallel in one side of a second direction intersecting with the first direction,

the light guide portion has a light-emitting portion provided at a position toward another end side that is away from the light-incident portion, and

wherein, when viewed from a third direction intersecting with the first direction and the second direction, the image light beams emitted from the same portion of the image generation system are converted into a parallel light flux by the light-incident portion, and the parallel light flux fills the inside of a portion of the light guide portion on the one end side.

2. The display apparatus according to claim 1,

wherein, assuming that a light flux diameter of the parallel light flux when viewed from the third direction is D, that a distance between the first surface and the second surface in the second direction is t, and that an incident angle of the parallel light flux with respect to the first surface and the second surface is θ, the light flux diameter D, the distance t, and the incident angle θ satisfy the following conditional equation.

D=2t×Sin θ

3. The display apparatus according to claim 1,

wherein, when viewed from the third direction,

the parallel light flux including:

a first light beam which is incident on the second surface and is positioned at one end portion of the parallel light flux;

a second light beam which is incident on the second surface and is positioned at another end portion of the parallel light flux; and

a center light beam which is positioned at center of the parallel light flux,

a position of the first light beam incident on the second surface is positioned on a normal line which is normal to the first surface and is extended from a position of the center light beam incident on the first surface.

4. The display apparatus according to claim 1,

wherein at least one of the light-incident curved surface and the light-reflective curved surface is a free curved surface.

5. The display apparatus according to claim 1,

wherein the light-reflective curved surface includes a reflective metal layer.

6. The display apparatus according to claim 1,

wherein the light guide portion includes a plurality of partial reflection layers which are disposed parallel to each other along the first direction,

each of the partial reflection layer of the plurality of partial reflection layers being inclined toward the one end side from a normal direction of the second surface when viewed from the third direction, and

wherein the light-emitting portion is a portion of the second surface that overlaps with the plurality of partial reflection layers in the second direction.

7. The display apparatus according to claim 6,

wherein the light-incident portion is formed of a first transparent member, and

wherein at least a portion of the light guide portion in which the plurality of partial reflection layers are formed is formed of a second transparent member which is surface-bonded to the first transparent member via a first bonding surface in the first direction.

8. The display apparatus according to claim 7,

wherein a first end portion of the first bonding surface on another side of the second direction is positioned closer to the one end side than a second end portion of the first bonding surface on the one side of the second direction is.

9. The display apparatus according to claim 7,

wherein the first bonding surface is positioned between the portion in which the plurality of partial reflection layers are formed and the light-incident portion.

10. The display apparatus according to claim 9,

wherein the first bonding surface is positioned between the light-incident portion and the light guide portion.

11. The display apparatus according to claim 9,

wherein the first bonding surface is positioned at a position overlapping with the partial reflection surface positioned closest to the one end side among the plurality of partial reflection layers.

12. The display apparatus according to claim 9,

wherein the first bonding surface includes a plurality of stepped portions.

13. The display apparatus according to claim 7,

wherein the first transparent member includes a plate-shaped portion overlapping with the light guide portion in the one side of the second direction, and

wherein the plate-shaped portion is surface-bonded to the light guide portion via a second bonding surface in the second direction.

14. The display apparatus according to claim 13,

wherein at least one partial reflection layer among the plurality of partial reflection layers includes a multilayer film including a reflective metal layer.

15. The display apparatus according to claim 13,

wherein the plurality of partial reflection layers have a characteristic in which a reflectance changes according to an incident angle of an incident light beam.

16. The display apparatus according to claim 15,

wherein each of the plurality of partial reflection layers has a larger reflectance when the incident angle is large than when the incident angle is small.

17. The display apparatus according to claim 15,

wherein each of the plurality of partial reflection layers has a larger reflectance when the incident angle is small than when the incident angle is large.